Regulation of ribosome accumulation is linked to the rate of cell growth both in eukaryotes and prokaryotes. In all organisms, the rate of ribosomal RNA synthesis is, in part, regulated by the level of precursor rRNA. We want to investigate how this is regulated in a mammalian cell, where the rDNA genes are neither amplified nor extrachromosomal. We propose three areas of study: 1) Experiments to define whether growth stimulation increases 455 rRNA synthesis by changing rRNA degradation, increasing frequency of chain initiation and/or by opening up new rDNA genes for transcription. 2) Structure of rDNA repeat unit, particularly near the transcriptional initiation site, in regard to primary sequence, level of cytosine methylation and nucleosome phasing. 3) Development of invivo assay for rDNA promoter function using cloned promoter DNA introduced into cells by infection with a recombinant SV40 vector or transfection.